1. Field of the Invention
The present invention relates to the growth of high quality SiC single crystals.
2. Description of Related Art
Silicon carbide is an important wide-bandgap material used for the development and manufacturing of semiconductor devices of new generation. Wafers of silicon carbide of 4H and 6H polytypes serve as lattice-matched substrates to grow epitaxial layers of SiC and AlGaN, which are used for the fabrication of SiC— and AlGaN-based devices. While AlGaN-based devices are intended for operation at microwave frequencies, SiC-based devices are aimed at efficient power switching. In order to produce low-defect epilayers and high-quality devices, the substrates of these devices must have good crystal quality, that is, they must contain low densities of inclusions, micropipes, sub-grains and other defects.
On the industrial scale, single crystals of silicon carbide are grown by sublimation. A schematic diagram of a SiC sublimation growth system is shown in FIG. 1. In the preparation for growth, a graphite crucible 1 is loaded with a polycrystalline SiC source 2 and a SiC single crystal seed 3. Typically, source 2 is placed at the bottom of the growth crucible 1, while seed 3, typically having the shape of a plate or wafer, is attached to a lid 4 of crucible 1, which lid 4 serves as a seed holder and is also made of graphite. Loaded crucible 1 is placed inside a gastight growth chamber 7. Then, crucible 1 is filled with an inert gas, such as, without limitation, argon or helium, in any suitable and/or desirable manner and heated to a growth temperature, which can be between 2000 and 2400° C., by a suitable heating means 8, such as, without limitation, an RF coil. Desirably, chamber 7 is made from fused silica. At growth temperature, source 2 vaporizes and fills the interior of the crucible 1 with a vapor 5 comprising volatile molecular species Si2C, SiC2 and Si. During growth, the temperature of source 2 is kept higher than the temperature of seed 3. This temperature gradient forces vapor 5 species to migrate toward seed 3 and precipitate thereon causing growth of a SiC single crystal 6. In order to control the growth rate and ensure high crystal quality, PVT growth is carried out in a flow of low pressure inert gas, such as argon or helium, inside crucible 1, generally at a pressure between 1 and 200 Torr. The flow of inert gas can be introduced into the crucible 1 in any suitable and/or desirable manner known in the art. For example, the flow of the inert gas can be introduced into crucible 1 via the porous walls thereof by way of a gas inlet 9 and a gas outlet 10 of sealed chamber 7.
From the standpoint of thermodynamics, two general cases are possible that describe equilibria within crucible 1 at high temperatures: (i) SiC is in equilibrium with carbon and (ii) SiC is in equilibrium with liquid silicon. Based on thermodynamic calculations, the composition of the vapor phase comprising Si, Si2C and SiC2 gaseous molecules in the SiC+C and SiC+Si systems are shown in FIGS. 2(a) and 2(b), respectively.
Sublimation growth of SiC single crystal 6 is generally carried out in a graphite crucible, such as graphite crucible 1. This creates a carbon-rich growth environment in which the pressure and composition of the vapor phase corresponds, generally, to the three-phase equilibrium between the vapor, SiC and C (shown in FIG. 2(a)). Under such conditions, simultaneous crystallization of SiC and C is possible and can lead to the formation of carbon inclusions in crystal 6. There are also other potential sources of carbon contamination of the growth interface and carbon inclusions in crystal 6. They include: (i) carbon particles that can be transported by the gas flow to the growing crystal 6 from the carbonized SiC source 2 and (ii) carbon particles that can be generated as a result of the erosion of the graphite of crucible 1 by the aggressive Si-rich vapor.
It is believed that the incidence of carbon inclusions in crystals, e.g., crystal 6, grown conventionally using the techniques of the prior art is quite high. A photograph showing a cross section of one of such 6H—SiC single crystal under ×25 magnification is shown in FIG. 3. The photograph of FIG. 3 shows inclusions near the seed-crystal interface, as well as trails of macro-defects originating from the inclusions. Chemical Auger analysis showed that these inclusions comprise carbon. In addition to macro-defects, the presence of carbon inclusions in the crystal bulk causes the appearance of microscopic defects, such as micropipes and dislocations.
It is believed that the presence of carbon inclusions can be reduced if the growth atmosphere is silicon rich with its composition corresponding to the SiC+Si equilibrium shown in FIG. 2(b). Adding elemental silicon to the SiC source was heretofore believed to be the most straightforward way to achieve this. Unfortunately, in the case of SiC+Si equilibrium, simultaneous crystallization of SiC and Si can take place leading to the appearance of silicon inclusions. In addition, an excess of silicon in the crystal growth charge, i.e., source 2, can lead to strong erosion of graphite parts. Finally, over-stoichiometric silicon lasts in the crystal growth crucible only for a short period of time due to the high vapor pressure over elemental silicon.
Accordingly, it would be desirable to find an additive to the crystal growth charge that does not alter significantly the SiC ratio in the vapor phase, but which would remove the carbon contaminants from the growth interface.